PROJECT SUMMARY/ ABSTRACT It has become accepted canonical dogma that granulocytes, such as eosinophils, play a deleterious and uniformly destructive role in lung allograft survival. In direct contrast to this notion our laboratory has made the surprising observation that eosinophils are essential for lung allograft tolerance. Specifically, we demonstrate that in the presence of co-stimulatory blockade immunosuppression the lung allograft is rapidly infiltrated by a high numbers of inducible nitric oxide synthase positive eosinophils. This eosinophil population plays a critical role in the downregulation of T cell responses and ameliorating graft rejection. Depletion of eosinophils prior to engraftment prevents co-stimulatory blockade-mediated lung allograft tolerance. These findings led to our central hypothesis that early recruitment of eosinophils into the lung leads to allograft immunosuppression through iNOS-dependent pathways. Towards this hypothesis we propose three Specific Aims to define the cellular mechanism used by eosinophils to induce graft acceptance. In Aim 1 we propose to define the physiology of iNOS production and mechanism/s of immunosuppression in the lung allograft through use of eosinophil-specific iNOS pathway deficient mice, characterization of NO/ROS modification of T cells, and regulation of iNOS by heme-oxygenase 1. In Aim 2 we will define the role of professional antigen presenting cells in eosinophil-mediated immunosuppression by measuring eosinophil-specific mediators of that suppress dendritic cell activity, bi-directional interactions of eosinophils and dendritic cells, and live imaging of synaptic formation between these cells with T cells. In Aim 3 we will define the migratory patterns that contribute to eosinophil-mediated tolerance. We will complete this through measuring the effect of co-stimulatory blockade on eosinophil chemokine eotaxin-2 as well as determine the critical time point of eosinophil recruitment to the lungs using inducible eosinophil depletion mice. Our data will provide novel insight into cellular immune responses contributing to lung allograft tolerance and may shed light on the design of novel therapeutic strategies that are not currently being explored.